Fuse



July 29, 1952 H H, FAHNOE 2,605,371

FUSE

Filed July l0, 1948 .1. QF'g n 25 38 Fig-2.

WITNESSES: INVENTOR Patented July 29, 1952 FUSE Harold H. Fahnoe,Wilkinsburg,/Pa., assignor to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvaniav Application July 10,1948, SerialNo. 38,031

(Cl. 200m-1,20)y

20Claims.

1 Y, This invention relates to electrical fuses which are adapted tointerrupt an electrical" circuit on overloads, and particularly toincreasing theability of such fusesA to withstand for short-times.moderate overloads, such asfmay be encountered in startingy motors orthe like, without ,materially alter-ing the rated current. at which suchfuses operateor thetime requiredfor their operation on heavier overloadssuch as those prompted vby. short-circuits.

lThis invention is of especial importance in fuses of the currentlimiting type,.such as those disclosed inthe copending application ofHarold H. Fahnoe, Serial No. 582,867, filed March 15, 1945, and issuedas Patent No. 2,496,704, dated February 7, 1950, on the-subject oflCircuit Interrupters. As explained in the copending applica tion justreferred to, most effective current and voltage limitation on vhighoverloads and shortcircuits is achieved whenthe overload is limitedbefore `maximum current prevails in the first half cycle'of theoverload. 1f limitation is not effected in this manner, the resultingsurge on the 'rst half cycle of faultv current often is capable of doingconsiderable damage to the circuit and especially'to connected apparatussuch as motors, transformers, or the like. It follows from this that thecurrent limitingvmeans importantly becomes effectiveina small fractionof a half-'cycle of such overloads as are tobe limited, thisk amountingrin time to a fewhundred microseconds.

A variety of the fuses referred' to in the copending application gaverise to a problem incident upon the assurance of rapid overloadlimitation, namely that of preventing` interruption of current clue tomoderate overloads, such as those of short duration which occur 'in'ystarting an electric motor followed by the resumption of normalrelatively low current conditions. By4

resorting to such measures vas adding more parallelfuse wires in thecurrent limiting element of the fuse to compensate for motor inrushstarting currents or the like, the peak current limit permitted by thefuseis correspondingly increased.

thus increasing the interrupting duty ofj the fuse,`

and the time-current characteristics on moderate short-circuits oroverloads is correspondingly' slowed down.

, It is, accordingly, an object of this invention' to provide a currentlimiting fusewhichis capab1eofwithstanding light overload `currents forshort periods of time, yet assures an extremely rapid blowing actionwhere relatively large over.- load currents are encountered.

Another object of this invention is the provision'of a currentlimitingfuse in which fusion of the fusible portion or portions thereofis reliablyand effectively Iarrested in a relatively low current rangeby heat absorption. and without substantial impairment of desired rapidlimitation of relatively large overloads.

A further object4 of thisv invention isthe provision of a fuse capableof permitting passage of inrush currents within lower. operating limitsand of interrupting relatively large overload currents, and which uponbeing subjected to asuiicient current overload to give the blowingaction, enables the rapid achievement of a high arc voltage contributingto the current limiting effect. y

Astill furthery object of this invention is to provide a fuse structureincluding fusible material, and heat absorbing means for cooperatingwith the fusible material, all supported in a novel andhighly usefulmanner interiorly of a tubular fuse holder.

These and other objects of this invention will..

Figure` 1 is asubstantially central longitudinal sectional view of afuse constructed in accord- .ance withv the invention, with certain ofthe parts showninxelevation;

Fig. V2 is .an enlarged, partial, transverse crosssectional view ofythefuse represented in Fig. 1, taken substantially on the line II--IIthereof;

Eig. 3 is an enlarged, partial, transverse crosssectional view. of` thefuse represented inFig. 1, this being takenr substantially on the lineIII-III thereof; and

Fig; 4is anenlarged, partial, longitudinal, sectional view taken throughone of the slotsor groovesv of the-fusesupporting member depicted inFig. 1'.

Iny the particular embodiment of the invention disclosed' in theaccompanying drawing, a limiting unit l0 kis provided for'limitingcurrentand voltage. This unit is encased `by a tubular fuse holder 2made of insulating material such as fiber or synthetic resin, and byterminal caps 4` and' 6y overy the-'opposite endsof the tube. Terminalcaps 4'- and 6=aremade of an electrically conductive material such,forfexample, as copper or` the like, and are preferably secured inposition on the tubular holder, illustratively by the staking pins'8.

The limiter unit I0 includes a supporting rodl I 2"'of suitableinsulating material suchi asffiber orv for example of brass, eachthreadedly engaging a corresponding one of the opposite inside Wall endsof the tubular fuse holder 2. Staking pins 8 advantageously reach intothese end plugs for stability. An electrically conductive rod end cap II, threadedly receiving one end of the rod I2, has a reduced diameterportion extending through aligned, substantially central openings in theend plug 25 and terminal cap 4. Similarly an electrically conductive rodend cap 56 threadedlv receives the other end of the supporting rod I2,and has a reduced diameter portion passing through aligned,substantially central openings in the end plug 26 and terminal cap 6.The reduced diameter portions of the rod end caps 25 and 26,conveniently are secured in electrically conductive relation to therespective terminal caps 4 and 6 as by soldering. A finely divided inertinsulating material 38, such as sand, surrounds the current and voltagelimiting unit II) and fills the space to the inside surface of tubularfuse holder 2 and of end plugs 25 and 26. The end plug 25 has a slightlytapered stopper 9 closing oif a suitable opening for insertion of theinert material 38. Terminal cap 4 is suiciently near the stopper toprevent the same from backing out of its related opening. The rod endcap 56 in the present embodiment is greater in length than rod end capII, for reasons which appear more fully hereinafter.

The actual current and voltage limiting and are extinguishing structureembodied in the unit I comprises a plurality of tubes disposed on thesupporting rod I2. Each of the tubes 20 has spaced grooves 22vrunninglongitudinally along the outside surface thereof for receiving a fusiblelength or section of wire 24. The grooved tubes are preferably made ofan electrically insulating material which is capable of evolving an arcextinguishing gas when in proximity to an electric arc, such for exampleas fiber or synthetic resin. Also, on the supporting rod I2, andpreferably in alternate or spacing relation with the grooved tubes 20,are a plurality of heat wells 4U, each including a tube 2 I, such ascopper or other suitable electrically conductive and heat absorptivematerial. The tubes 2i) and 2| as arranged in the present embodiment areeasily tightened toward and clamped against each other in substantiallycoaxial relation on supporting rod I2 by advancing the rod end caps and56 relative to one another. Grooves 22 are longitudinally alignedthroughout the tubes 20 before tightening the end caps, or may be cut inalignment after thetightening operation. The bottoms of grooves 22conveniently are at about the same level as the outside surfaces of heatwell tubes 2i.

The fusible Wires or elements 24 extend in parallel electrical relationto each other from rod end cap 25 to the rod end cap 56, and each wirehas fusible sections individually in the grooves of the tubes 20 whichare connected in series with the heat wells 40. For this purpose wires24 conveniently pass along in the grooves 22 and over the surface of theheat Well tubes 2|, there being connected in heat conductive andelectrically conductive relation to the Well tubes as by an embeddinglayer of silver solder or the like 28. A plurality 0f Washers 34 areslipped onto each of the grooved tubes 20 and are frictionally held inspaced` relation to each other along the length of these tubes, forapurpose to be described. Washers 34 may be of any desired electricalinsulating material, such, for example, as fiber or the like.

An additional mass of heat absorptive material for the heat wells 40,supplementing the absorptive effect of the tubes 2|, convenientlysurrounds the solder bed 28 containing the fusible Wires 24. This mass,as represented in the accompanying drawing, includes a Wrapping of heatconductive wire over or in each of the solder beds 28 for reinforcingand strengthening purposes, and a superimposed segmented collar I5 ofthermally absorptive material which may be the same as the material ofsleeve 2|, is fastened by means of a layer of solder 39, for example ofsilver, to the Wire wrapping. Both the segmented collar I5 and theWrapping wire 30 illustratively are made of copper. The segmented collarconveniently provides a circumferential groove for receiving a bindingWire I8 which assists in holding together the several soldered segmentsboth before and after the soldering operation.

The fuse wire 24, being of silver or similar material, has a hightemperature coefficient of resistance, and is initially chosen to carrythe rated current without undue heating, and yet melt on currents abovethe rated current. Such a wire is then provided with a plurality ofreduced portions 36 (Fig. 4). By this arrangement the fusion time, atleast for high values of current is speeded up and substantiallycorresponds to the fusion time required by a wire having about the samediameter as the reduced diameter portions 36. This speeding up effecthas been found to be roughly proportional to the ratio between the areaof the large portions of the fuse wire 24 and the reduction portions 36,the ratio being according to cross section. As a practical matter, thisratio may be made as high as possible, being limited of course bymechanical strength considerations. Such a fuse wire retains all of thenormal time-current characteristics desired in a fuse for time values inexcess of approximately two cycles without appreciable reduction incurrent carrying ability. Reduction in current carrying ability variesapproximately in direct proportion to the length of portions 36, so thatit is desirable to make these portions as short as possible. For shorttimes of less than about one-fourth cycle, fuse wire 24 has anexceptionally fast melting time when the overload current is high. Whenthe overload current is relatively low, however, the heat Wells 40absorb enough of the heat to prevent the fuse wires from fusing. Thisfeature givesy ample protection against blowing of the fuse wires, asfor example when the related electrical circuit is carrying a motorstarting or inrush current before normal operating currents areattained. The effect of increased or sustained overloads, such as shortcircuits is that of limiting the current and voltage by fusion of theWires in an early part of the cycle of fault current. Under the latterconditions the heat wells 40 have substantially no arresting affect onfushion of the wires 24 and the melting time of less than aboutone-fourth half cycle.

As pointed out in the copending application hereinbefore referred to,the fuse wires 24 retain their current carrying ability Where thecurrent is relatively low, and this is not substantially impaired by theintroduction of reduced portions 36. Added to this in the presentembodiment, is the effect of the heat wells, which extends theshort-time current carrying ability of the fuse Wire to includeappreciably increased overloads, such for example as currents ranging upto about eight times the rated current, all with retention of fastlimiting characteristics for higher overloads.

Theactual limitation of current is achieved by.

ofthe fusible sections or lengths of thewire represented by thelength ofthe related grooved tube 2li. substantially instantaneously, thusexerting a limiting or choking effect on the current and preventing riseof the current beyond a predetermined value. The particular structuredisclosed provides a'high arc voltage per unit of length of the fusiblesections of wire 24, which interconnect' the wells 40 and lieintermediate the latter in the grooves 22 of tubes 20. rI'hisconveniently is achieved by employinga large enough number of reducedportions 3G in the fusewire so that when these reduced portions melt, `aplurality of serially related arcs may be formed and the summation ofthe arc voltages of these series arcs will be greater than the arcvoltage across asingle arc. Generally speaking, the arc voltage per unitof length is directly proportional to the number of series arcsorrestricticns per unitof length which may be effectively interrupted byfusion. This suggeststhat maximum arc voltage per unit of length may beattained by providing as many restrictions 36 perunit oflength as is:

physically possible. However, as the number of restrictions 36 per unitof length is increased, a point is eventually reached wherev a furtherincreaser results in a decrease in current carrying ability. It has beenfound that at least for voltages above 600 volts, at least 4tworestrictions 38 per effective inch of fuse wire 24, or a total of aboutten shouldv be provided to obtain an effective rise in arc voltage witha fuse wire `which is not yof excess length. This correspondsapproximately to a spacing ofy restrictions 36 not to exceed aboutfifteen times the large diameter of the fuse wire. t is desirable,however, Lto use the maximum number of restrictionsu possible Without.substantially impairing .the current carrying ability. The mostdesirable number of restrictions is dependent upon the size of the fusewire, and appears to correspond roughly toa spacing of about three and`one-half times-the largest diameter of the wire. This is. an optimum`spacing, since spacings as low as about twice thelargest diameter ofthefuse wiremay be employed with good results.

Another reason why the arcvoltage is rapidly built up to a value highenough to exert a current limiting effect, has to do with the grooves'22in which the fuse Wiresare located. lGrooves 22 advantageously aremadeas narrow. as possible, being limited only by the size of the fusewires, and in practice illustratively are made nomore than 0.010 inchlarger than the fuse wires. Preferably the grooves arey somewhat deeperthan the diameter of the fuse Wires, so that the yWires will vcloselycontact the groove on three sides and thus physically restrict the arcformed upon melting of the fuse wires, vthereby raising the arc voltageat the instant the arc is formedf Since fuse wires 24 are in closeproximity .tothe Walls of groove 22, the arc formed upon inter--`ruption will cause the evolution of arc-extinguishing gas from the wallsof the groove, and this gas will blast laterally through the arc toperform three functions, all of which act to furtherincrease the arcvoltage and extinguish the arc. First, the blast of arc-extinguishinggas acts to. sweep metal vapor out of thearc stream and out of grooves22 into the material'38 in which the metal vapor becomes dispersed andUpon fusion, a high'arc voltage Ais built up.:k

condensed into .separated particles insulated fromUr each other, sothat'a `high resistance path `is maintained outside the grooves.Secondly, the

blast of varc.-extinguishing gas also acts to supply.;

url-ionized gas to :the arc ypath to further increase.

the resistance oftheaarc path. and toiextinguish. r` A third functionof. the' transverse gas blast is to cause the series arcs-to.

the arc at current zero.

be looped-outwardly toward. filling material 38, thereby lengthening thearc path and, consequently, increasing the resistance'fthereof andthevoltage drop across it. A function of washers 34 on supporting rod` 20will nowv be readily under-1 stood as preventing escape ofy the arcsfrom grooves 22 and'y consequently, keeping the aresfin prOXimityWiththe'gas evolving material and the restricting action of the narrow.grooves, andl structure disclosed, because (1) the fusible wireslthemselves are capable of melting toestablish an.

arcvat least on such high currents which it is desired to limit, in avery short time, that is, in a very small fraction of` a half cycle, and(2) as soon as the arcis established. it is subjected to all ofthefactors enumerated above to create an arc voltage high enough to preventany further rise of fault current. f

The; arc voltage, and the speed and efficiency of current limitation andinterruption appear to be in some measure dependent upon ythe number andthickness of washers 34, vThese washers are `preferably 4made thickenough. as not to be materially eroded by the action of the arc. thenumber of washers v34 increases the arc volt.- age while increasingcurrent limitation and interruptionspeed. A better understanding of thismay be arrived at by considering that fusesv are known in the art whichcomprise one or more silver wires embedded in sand and that these fusesobtain currentlimitation by the formation of a semi-conductive coreabout the wire due to the action of the heat of the arc on the sand.However, such fuses must be excessivelyy long in order to obtainsufficient voltage drop to extinguish the arc, and increased length, ofcourse, causes greater voltage surge at interruption. In the fuseconstituting this invention the formation of any such semi-conductor ispositively prevented by barrier plates 3.4, and this is one reason whyunit I0 may be made relatively shortin length. The provision of barrierplates 34 (which themselves. are advantageously of a material capable ofevolving an arcextinguishing gas when subjected to the heatxof an arc)thus appears to provide spaced deionizing zones along the length of unitil), which would explain why an increase in the number of barriers wouldprovide an increase in arc voltage,-and also explains why a relativelyhigh arc voltage per unit length is obtained with unit l2. Moreover, itis believed that this action of the barrier plates 34 undoubtedlycontributes to rapid extinction of the arc. 1

Though there may be other reasons for the results obtained, tests showthat a structure such as unit I0 of and by itself, is capable oflimiting high overload currents in the first part of the rst half cyclethereof, without producing voltage surges substantially greater` thanone and one-half times restored circuit voltage, and

It also appears that an increase :inr

is effective to interrupt the circuit at the following first or secondcurrent zero.

Further, the unit is thoroughly capable of absorbing heat resulting fromrelatively low overload currents of short duration, thus preventinginterruption of the circuit on such overloads. The number and size ofthe heat wells 4D are correlated with the length of fuse wires 2:4intermediate the wells so that the entire heat input due to momentarylight overloads can be conducted to the wells before the centers of thefuse wires attain fusion temperature. Furthermore, a material such ascopper is chosen for the heat wells because its specific heat is higher(about twice as high) than the material of which they fuse wires areformed. This means that for a given size of heat well a relatively largeamount of heat can be stored therein. This heat lost from the wires towells.

40, as well as some losses to the surrounding media, such as sand 38 androd l2, thus limits the temperature rise of fuse wires 24 on lightoverloads of short duration. In addition, since wells 4D electricallyand mechanically connect all of the parallel wires, increased uniformityin heating and in division of current flow through the several fusewires is achieved. Moreover, since wells 40 effectively break up eachfuse wire 24 into a plurality of serially arranged short fusiblesections located between the wells, longitudinal expansion andcontraction of the wires is not a factor in causing fuse wire bending oreven breakage occurring in the fuses which employ longer lengths of fusewire. One reason why peak currents on heavy overloads are not increasedwith the presen design is that fuse wires 24 intermediate wells 4G arenot increased in number, and they act when the fuse blows on such heavyoverloads in exactly thesame manner as if wells 40 were not present.

To give an indication that fusion of the current limiting wires 24 hasoccurred in the unit I0, an indicator disc 68 is provided secured to aconnecting rod 5B extending through a bore in the outer end of the rodend cap 56, to be normally held adjacent to end terminal cap 6 by a highresistance fusible strain wire 48 and a small tension spring 52. A coilcompression spring 1U is provided between indicator disc 68 and cap 6 tobias the disc outwardly away from the end of the fuse. The large endcoils 46 of spring 52 are adapted to be supported within fuse tube 2 andmore particularly within the bore of rod end cap 56 by a shoulder in asmall inner tube 44 of insulating material such as fiber or the like.The inner end of strain wire 48 is connected through spring 52 to therod end cap Il by means of a high resistance fusible wire 66. Highresistance wires 48 and 6U preferably are of such a high resistance ascompared with fuse wire 24 that they normally do not carry anyappreciable amount of current. However, as soon as wires 24 are melted,the arc voltage developed in unit I0 forces sufficient current throughwires 60 and 48 to fuse these wires at substantially thel same time,thus releasing indicator disc 68 and permitting it to be moved by springi6 outwardly to a clearly visible indicating position. Fusible wires 48and 69 are of such high resistance relative Ato wires 24, on the orderof several hundred times the resistance of wires 24, that they `do notaffect the operation of unit (D in any appreciable way either before orduring a circuit interrupting operation. l

The fuse construction described herein provides a simplified form offuse which may be maderelatively small in size. It includes both currentand voltage limiting means in a single unit, together witharc-extinguishing means.

, The limiting unit I0 has a definite, continuous current rating and thefusible sections of wires 24 will rapidly fuse on high overload currentswhich it is desired to limit, but will resist fusion of lower overloadcurrents up to about 800 per cent of rated current and of short durationby aid of the heat Wells 4U, the latter also being compactly containedin the limiting unit I0.

Having described a preferred embodiment of the invention in accordancewith the patent statutes, it is desired that the invention be notlimited to this particular structure, inasmuch as it will be obvious topersons skilled in the art that many modifications and changes may bemade in this particular structure Without departing from the broadspirit and scope of this invention. Accordingly, it is desired that theinvention be interpreted as broadly as possible and that it be limitedonly as required by the prior art.

I claim as my invention:

1. A fuse having a definite, continuous current rating, comprising atleast one fusible element having a maximum crosssection area notexceeding that which will fuse on currents in excess of rated current, arelatively large quantity of. good heat conducting material located ingood heat conducting relation with said fusible element to preventfusion thereof within a higher range of currents of short duration, andsubstantially all of said material having a relatively high specificheat.

2. A fuse having a definite, continuous current rating, comprising,serially arranged fusible elements, each of said elements having amaximum cross-section area not exceeding that which will fuse oncurrents in excess of rated current. a relatively large mass of goodheat conducting material located at each pair of adjacent ends of saidelements to prevent fusion thereof within a higher range of currents ofshort duration, and Substantially all of said material having arelatively high specific heat.

3. A fuse having a definite, continuous current rating, comprising aplurality of spaced parallel fusible elements each of which comprisesserially arranged fusible sections, each of said sections having amaximum cross-section area not exceeding that which will fuse oncurrents in excess of rated current, and relatively large masses of goodelectrical conducting material each of which is common to andelectrically connects corresponding sections of said elements,respectively.

4. A fuse having a definite, continuous current rating, comprisingserially arranged fusible sections, each of said sections having amaximum cross-section area not exceeding that which will fuse oncurrents in excess of rated current and portions having a reducedcross-section area along the length thereof to substantially decreasemelting time thereof on heavy overload; and at least one heat wellinterposed between said sections in heat conducting relation thereto toprevent fusion thereof within a range of currents of short duration.

5. A fuse having a denite, continuous current rating, comprising, aplurality of parallel fusible elements each of which comprises seriallyarranged fusible sections, each of said sections having a maximumcross-section area not exceeding that which will fuse on currents in`excess ofrated current and portions having a reduced cross-section areaalong the'length thereof to substantially decrease melting time thereofon heavy overload; and heat wells between said sections in heatconducting relation thereto to prevent fusion thereof within a range ofcurrents of rvshort duration, with the heat wells between corresponding'sections of said elements comprising single unitary commonl masses ofheat conducting material, respectively. i

` of short duration, with said heat Well comprising a single unitarymass of heat conducting material common to all of said elements.

'1.A fuse having a definite, continuous current rating, comprising, atleast one elongated fusible section having a maximum cross-section areanot exceeding that which will fuse on currents in excess of ratedcurrent and portions having a reduced cross-section along the lengththereof to substantially decrease melting time thereof on highoverloads, and a sufficient large quantity of good heat conductingmaterial located in heat conducting relation with said fusible sectionto prevent fusion thereof within a range of currents of short duration.

8. A fuse having a definite, continuous current rating, comprising, atleast one elongated fusible section having a cross-section area notexceeding that which Will fuse on currents in excesss of rated currentand portions having a reduced cross-section area along the lengththereof to substantially decrease melting time thereof on highoverloads, a suiiiciently large quantity of good heat conductingmaterial located in heat conducting relation with said fusible sectionto prevent fusion thereof within a range of currents of short duration,and an elongated arc restrictive passage of insulating materialreceiving said fusible section for increasing the f arc voltage alongthe same under conditions of relatively high overload current.

9. A fuse having a definite, continuous current rating, comprising,serially arranged fusible sections, each of said sections having acrosssectionarea not exceeding that which will fuse on currents inexcess of rated current and portions having a reduced cross-section areaalong the length thereof to substantially decrease melting time thereofon high overload, heat wells between said sections to prevent fusionthereof within a range of currents of short duration, and insulatingmaterial adjacent said fusible sections capable of evolving an arcextinguishing gas for increasing the arc voltage under conditions ofrelatively high overload current.

l0. A fuse having a definite, continuous current rating, comprising, aplurality of parallel fusible elements each of which comprises seriallyarranged fusible sections, each of said sections having a cross-sectionarea not exceeding that which will fuse on currents in excess of ratedcurrent and portions having a reduced cross-section area along thelength thereof to sustantially decrease melting time thereof on highoverload, heat wells between said sections f to prevent fusion thereofwithin a range of currents vof lshort duration, and elongated arcrestrictive passages of insulating material receiving vsaid fusiblelsections for increasing the arc voltage along the same under conditionsof relatively high overload current.

l1. A fuse comprising, at least two spaced parallel elongated fusiblesections, a single mass of heat conducting material located in heat`conducting relation with both of said fusible sections to preventfusionthereof within a range of currents of short duration.

12. A fuse comprising, at least two spaced parallel elongated fusiblesections, a mass of good electrical and heat conducting materialelectrically and mechanically connecting said fusible sections toprevent fusion thereof within a range of currents. of'short durations13. A fuse comprising, elongated fusible sections, spaced longitudinallygrooved elements receiving said lfusible sections in the groovesthereof, for increasing the arc voltage along the same under conditionsof relatively high overload current, arc escape prevention meanslongitudinally spaced along said grooved elements and the groove pathsthereof, and heat absorptive means between said grooved elements in heatconducting relation with said fusible sections to prevent fusion thereofwithin a relatively low range of currents of short duration.

14. A fuse comprising, elongated fusible sections, spaced longitudinallygrooved elements receiving said fusible sections in the grooves thereof,for increasing the arc voltage along the same under conditions ofrelatively high overload current, arc escape prevention meanslongitudinally spaced along said grooved elements and the groove pathsthereof, and good electrical and heat conducting means located betweensaid grooved elements and being in electrical and heat conductingrelation with said fusible sections to prevent fusion thereof Within arelatively low range of currents of short duration.

15. A fuse comprising, a plurality of parallel fusible elements each ofwhich comprises serially arranged fusible sections, spaced substantiallytubular heat wells in heat conducting relation with correspondingfusible sections of all of the fusible elements to prevent fusionthereof within a relatively low range of currents of short duration,substantially tubular elements between said heat wells and havinglongitudinal external grooves for receiving said fusible sections andincreasing the arc voltagealong the same under conditions of relativelyhigh overload current, arc escape prevention washers longitudinallyspaced on said grooved tubular elements and over the groove pathsthereof, and a rod extending longitudinally through said heat wells andgrooved tubular elements for supporting the same as a unit.

16. Afuse comprising, a plurality of spaced parallel fusible elements,means at corresponding ends of said elements for electrically connectingthem in parallel circuit relation, and a plurality of heat wells spacedalong the length of said elements, each of said wells comprising aunitary mass of good heat conducting material extending into heatconducting relation with corresponding points of all of said elements.

17. A fuse comprising, a plurality of spaced parallel fusible elements,means at corresponding ends of said elements for electrically connectingthem in parallel circuit relation, and a plurality of means ofelectrical conducting material spaced along the length of said elements,each electrically connecting corresponding points of all of saidelements.

18. A fuse comprising, a plurality 'of spaced parallel fusible elements,means at corresponding ends of said elements for electrically connectingthem in parallel circuit relation, and a plurality of heat Wells spacedalong the length of said elements, each of said Wells comprising aunitary mass of good electrical and heat conducting material extendinginto electrical and heat conducting relation withl corresponding pointsof all of said elements.

19. A fuse comprising, an elongated fusible element, supporting meansfor said element comprising spaced sections of insulating material andinterposed sections of good electrical and heat conducting materialarranged longitudinally of said element and mechanically engaging thesame.

20. A fuse comprising, a plurality of spaced,

REFERENCES CITED The following references are of record in the nie ofthis patent:

UNITED STATES PATENTS Number s Name Date 2,004,191 Bussmann June 11,1935 2,055,866 Jung et al Sept. 29, 1936 2,209,823 Lohausen July 30,1940

